1. Field of the Invention
The present invention relates to an organic electroluminescent (hereinafter, also referred to as an organic EL) display device and a production method thereof. More specifically, the present invention relates to an organic EL display device including an organic EL layer which is formed by a wet method such as an ink-jet method. The present invention further relates to a production method of such an organic EL display device.
2. Description of the Related Art
An organic EL display has been drawing attention as a next-generation flat panel display (FPD) because such a display is excellent in visibility such as a viewing angle and responsiveness and permits low power consumption, slim profile, and flexibility of the display itself. Such an organic EL display device is still inferior to a liquid crystal display (LCD) or a plasma display panel (PDP) in technical completeness or standard of industrial infrastructure. Therefore, practical use of the organic EL display device is still only loading on car audios or some mobile information devices. However, the organic EL display device is theoretically the most excellent FPD and therefore, future market expansion is expected for the display device.
Such an organic EL display device displays images by driving an organic EL panel having a configuration in which a light-emitting organic EL element is arranged in every pixel. The organic EL element has a structure in which an organic EL layer including a light-emitting layer is interposed between a pair of electrodes at least one of which has translucency. When a voltage is applied to the light-emitting layer, the organic EL element emits light. In addition to the light-emitting layer, a layer whose conductivity is higher than that of the light-emitting layer, such as a hole injection layer, is normally arranged as the organic EL layer in order to improve a light-emitting efficiency.
A low-molecular organic EL layer and a polymer organic EL layer are known as the organic EL layer. A method of forming a film by vacuum deposition is commonly used to form the low-molecular organic EL layer, but an uneven film tends to be formed by this method. Therefore, it is difficult to form a large organic EL layer by this method. In contrast, a wet method such as an ink-jet method, a nozzle coating method, and a printing method is commonly used to form the polymer organic EL layer. Among these, a method in which RGB pixels are pattern-formed on a substrate by an ink-jet method has been widely used, recently (for example, refer to Japanese Kokai Publication No. Hei-10-12377, Japanese Kokai Publication No. Hei-10-153967, and Japanese Kokai Publication No. 2002-334782).
However, the organic EL element prepared by the wet method generates a leakage current which does not contribute to light emission when a voltage is applied, which adversely influences the light emitting characteristics.
For this problem, an organic EL element in which a region where a light-emitting layer is formed is the same as or larger than a region where a hole injection/transport layer is formed is disclosed (for example, refer to WO 01/074121). Such an organic EL element generates no leak current and has excellent characteristics such as a high light-emitting efficiency. However, even if the light-emitting layer is formed to completely cover the hole injection/transport layer, a leakage current must be generated. Therefore, there is room for improvement in a reduction in power consumption and improvement in light-emitting characteristics.